1. Field of the Invention
The invention relates to an electric motor power source for an electric traction drive including a current limiter.
2. Background Art
There is a variety of vehicle powertrains that employ a high voltage battery and an electric driving motor as a primary source of power. If an internal combustion engine is included in the powertrain in combination with the electric driving motor, the powertrain usually is referred to as a hybrid powertrain. An example of a hybrid vehicle powertrain is disclosed in U.S. Pat. No. 6,176,808, which is assigned to the assignee of the present invention. The powertrain of the ""808 patent includes an induction motor that complements engine torque to provide improved launch performance. The engine may be deactivated when the vehicle is at rest.
A hybrid powertrain can be characterized as a series hybrid powertrain or a parallel hybrid powertrain or a combination of both. In a series hybrid powertrain, a generator is driven by an engine, and the generator, together with a battery, power a vehicle driving motor. The battery is charged by the generator. The engine can be mechanically coupled to the driving wheels during an engine driving mode. In a parallel hybrid powertrain, the engine is coupled to the output shaft of an electric motor. The output torque of the engine and the output torque of the motor are then supplied along parallel torque flow paths to the driving wheels. An example of a hybrid vehicle powertrain of this kind is disclosed in U.S. Pat. No. 5,635,805.
A parallel hybrid electric vehicle powertrain has a split power flow path. The motor torque and the engine torque are combined using a geared transmission system, the battery system providing energy storage for the generator and motor.
In hybrid electric vehicle powertrains, as well as in pure electric vehicle powertrains, the power source requires a high voltage battery, which is a major cost component for the powertrain. Because of the cost, the overall life expectancy and durability of the battery present a major issue in any evaluation of the feasibility of these kinds of vehicles. The battery, furthermore, requires periodic recharging. One of the limitations of the battery charging procedure is the possibility of developing excessive temperatures. Excessive temperatures may result also if the battery is discharged at an excessive rate.
All of the operating conditions for the battery are continuously monitored in a typical hybrid electric vehicle powertrain. These conditions are distributed to a vehicle control system, which must be calibrated to protect the battery from developing excessive temperatures due to overcharging or discharging, thereby preserving battery life.
The invention provides precise control of the electric motor torque demand on the battery""s subsystem. The greatest power demand typically is the electric traction drive. The invention provides improvement in the life and durability of the battery by precisely controlling the demands on the battery subsystem. A battery current limiter is included in the control system of the invention for continuously monitoring the battery conditions. That monitored information is used to reduce the torque produced by the electric drive. This, in turn, limits the battery current, thereby ensuring increased battery life.
The method and current limiter of the invention monitors battery system conditions, which are delivered to a vehicle controller. A torque command is developed by the vehicle controller and delivered to an electric drive motor system, which includes a power converter and the electric motor, whereby electric driving power is developed. A feed-forward torque signal, which is a function of commanded torque, maximum current limit and a calibrated margin current, is delivered in a feed-forward fashion to the vehicle controller whereby a modified torque signal is developed. This torque signal is an estimate of the amount of torque reduction needed to prevent the commanded torque from exceeding a commanded torque corresponding to a maximum current limit.